FIG. 4 shows a typical polymerization reactor used for producing a polymer such as a water absorbent resin. The illustrated polymerization reactor has a container body 91 for containing e.g. a reaction solution, and an outer shell (jacket 92) that covers the outer surface of the container body 91. A cooling/heating medium (cooling medium or heating medium) is caused to flow through the inside of the jacket 92 as required, so that the content (e.g. reaction solution or reaction mixture) is cooled or heated due to heat transfer through the wall of the container body 91. Thus, the temperature of the content of the container body 91 is controlled to a predetermined temperature. The container body 91 needs to have a certain degree of strength to withstand the load of the content and the pressure during the polymerization process. Thus, the container body 91 is made of a material having excellent strength (e.g. carbon steel plate). Generally, the container body 91 is provided with an agitating element 93 for making the content homogeneous and have a uniform temperature. For instance, a polymerization reactor including a container body and an outer shell is disclosed in Patent Document 1 identified below.
In recent years, there is an increasing demand for polymers such as a water absorbent resin. To respond to such an increasing demand, it is considered to enhance the productivity by increasing the size of a polymerization reactor.
To control the content to a desired temperature in producing a polymer by using a polymerization reactor, a relatively large amount of heat needs to be transmitted by heat transfer with a cooling/heating medium. Thus, in the process of producing a polymer (hereinafter referred to as “polymerization process”), the time taken for heating or cooling the content (hereinafter referred to as “heat transfer time”) accounts for a relatively large fraction. This indicates that shortening the heat transfer time can lead to enhancement of polymer production efficiency. To shorten the heat transfer time, the above-described heat transfer by heat exchange needs to be performed efficiently.
In flowing a cooling/heating medium inside the jacket, the following three are typical factors that can influence the heat transfer efficiency (heat transfer quantity Q) between the cooling/heating medium and the content through the container body. The first factor is the heat transfer resistance between the cooling/heating medium and the wall of the container body (e.g. the flow rate of the cooling/heating medium within the jacket). The second factor is the heat transfer resistance between the wall of the container body and the content (e.g. the degree of homogeneity and temperature uniformness of the content provided by the agitating element). The third factor is the heat transfer resistance of the metal of the wall of the container body itself. With respect to the first factor, the heat transfer efficiency can be improved by increasing the flow rate of the cooling/heating medium within the jacket. With respect to the second factor, the heat transfer efficiency can be improved by enhancing the homogeneity and temperature uniformity of the content provided by the agitating element. With respect to the third factor, the heat transfer efficiency can be improved by reducing the thickness of the wall of the container body. Of these three factors, the one that most influences the overall heat transfer efficiency is the third factor (metal resistance). Thus, reducing the thickness of the wall of the container body is considered to be the most effective way to shorten the heat transfer time.
However, to make a polymerization reactor larger to enhance the productivity as noted above, the container body needs to have a relatively large thickness in view of the strength. Such a large thickness leads to a decrease in the efficiency of heat transfer through the wall of the container body and hence leads to an increase in the heat transfer time, hindering the improvement of productivity.
In view of these circumstances, the structure disclosed in Patent Document 1 includes a passage for a cooling/heating medium on the inner side of the container body. Specifically, as shown in FIGS. 1 and 2 of this document, support members each comprising a strip-like plate are welded to the inner surface of the container body in an upright posture on the inner surface at predetermined intervals, and bonding plates each having a dimension corresponding to the interval between adjacent support members are welded to the ends of the support members. With this arrangement, the closed space defined by the wall of the container body, the support members and the bonding plates provides a passage for a cooling/heating medium. Providing a passage for a cooling/heating medium on the inner side of the container body in this way allows reducing the thickness of the bonding plates (i.e., the wall between the content and the cooling/heating medium), which leads to a shorter heat transfer time and an improved productivity.
However, it is difficult to check the condition inside the container body during the use of the polymerization reactor. Thus, in view of possible breakage or the like of the wall due to deterioration with time, reducing the wall thickness of the passage for a cooling/heating medium provided on the inner side of the container body and bonding the wall by welding causes poor reliability for long use.
Polymerization of water-soluble ethylenically unsaturated monomer is a mainstream method for producing a water absorbent resin. Water-soluble ethylenically unsaturated monomer is an acid substance and generally used after neutralization with sodium hydroxide. In producing a water absorbent resin by reversed phase suspension polymerization, an organic solvent is used as a reaction solvent. Since an acid, an alkali and/or an organic solvent are used in this way for polymerization reaction to produce a water absorbent resin, use of a corrosion-resistant metal such as stainless steel as the material for a polymerization reactor (container body) may be considered to be desirable. However, stainless steel is inferior to carbon steel in heat transfer ability. Thus, in the case where heating and/or cooling of the reaction solution (content) are performed by flowing a cooling/heating medium in the jacket 92 provided on the outer side of the container body in the structure shown in FIG. 4, the container body 91, when made of stainless steel, provides lower efficiency of heat transfer through its wall and the resulting longer heat transfer time, as compared with when the container body is made of carbon steel.